Since the demonstration of the first organic solar cell having an efficiency in the percent range by Tang et al. 1986 [C. W. Tang et al. Appl. Phys. Lett. 48, 183 (1986)], organic materials have been investigated intensively for various electronic and optoelectronic components. Organic solar cells consist of a sequence of thin layers (typically 1 nm to 1 μm) composed of organic materials, which are preferably applied by vapor deposition in a vacuum or by spin-coating from a solution. The electrical contact-connection can be effected by metal layers, transparent conductive oxides (TCOs) and/or transparent conductive polymers (PEDOT-PSS, PANI).
A solar cell converts light energy into electrical energy. In this case, the term photoactive likewise denotes the conversion of light energy into electrical energy. In contrast to inorganic solar cells, in organic solar cells the light does not directly generate free charge carriers, rather excitons initially form, that is to say electrically neutral excitation states (bound electron-hole pairs). It is only in a second step that these excitons are separated into free charge carriers which then contribute to the electric current flow.
The advantage of such organic-based components over the conventional inorganic-based components (semiconductors such as silicon, gallium arsenide) are the in some instances extremely high optical absorption coefficients (up to 2×105 cm−1), thus affording the possibility of producing very thin solar cells with little outlay in terms of material and energy. Further technological aspects include the low costs, the possibility of producing flexible large-area components on plastic films, and the virtually unlimited possibilities for variation and the unlimited availability of organic chemistry.
One possibility for the realization of an organic solar cell that has already been proposed in the literature consists in a pin diode [Martin Pfeiffer, “Controlled doping of organic vacuum deposited dye layers: basics and applications”, PhD thesis TU-Dresden, 1999] having the following layer construction:
0. carrier, substrate,
1. bottom contact, normally transparent,
2. p-layer(s),
3. i-layer(s),
4. n-layer(s),
5. top contact.
In this case, n and p denote an n-type and p-type doping, respectively, which lead to an increase in the density of free electrons and holes, respectively, in the thermal equilibrium state. However, it is also possible for the n-layer(s) and p-layer(s) to be at least in part nominally undoped and to have preferably n-conducting and preferably p-conducting properties, respectively, only on account of the material properties (e.g. different mobilities), on account of unknown impurities (e.g. residual residues from the synthesis, decomposition or reaction products during the layer production) or on account of influences of the surroundings (e.g. adjacent layers, indiffusion of metals or other organic materials, gas doping from the surrounding atmosphere). In this sense, layers of this type should primarily be understood as transport layers. By contrast, the designation i-layer denotes a nominally undoped layer (intrinsic layer). In this case, one or a plurality of i-layers can consist layers either composed of one material, or a mixture composed of two materials (so-called interpenetrating networks or bulk heterojunction; M. Hiramoto et al. Mol. Cryst. Liq. Cryst., 2006, 444, pp. 33-40). The light incident through the transparent bottom contact generates excitons (bound electron-hole pairs) in the i-layer or in the n-/p-layer. Said excitons can only be separated by very high electric fields or at suitable interfaces. Sufficiently high fields are not available in organic solar cells, with the result that all promising concepts for organic solar cells are based on the separation of excitons at photoactive interfaces. The excitons pass by diffusion to such an active interface, where electrons and holes are separated from one another. In this case, the material which takes up the electrons is designated as acceptor, and the material which takes up the hole is designated as donor. The separating interface can lie between the p- (n-) layer and the i-layer or between two i-layers. In the built-up electric field of the solar cell, the electrons are then transported away to the n-region and the holes to the p-region. Preferably, the transport layers are transparent or largely transparent materials having a large band gap (wide-gap) such as are described e.g. in WO 2004083958. In this case, the term wide-gap materials denotes materials whose absorption maximum lies in the wavelength range of <450 nm, and is preferably <400 nm.
Since the light always generates excitons first, and does not yet generate free charge carriers, the diffusion of excitons to the active interface with little recombination plays a critical part in organic solar cells. In order to make a contribution to the photocurrent, it is necessary, therefore, in a good organic solar cell, for the exciton diffusion length to distinctly exceed the typical penetration depth of the light, in order that the predominant part of the light can be utilized. Organic crystals or thin layers that are perfect structurally and with regard to chemical purity do indeed fulfill this criterion. For large-area applications, however, the use of monocrystalline organic materials is not possible and the production of multilayers with sufficient structural perfection is still very difficult to date.
If the i-layer is a mixed layer, then the task of light absorption is undertaken by either only one of the components or else both. The advantage of mixed layers is that the excitons generated only have to cover a very short path until they reach a domain boundary, where they are separated. The electrons and holes are respectively transported away separately in the respective materials. Since the materials are in contact everywhere with one another in the mixed layer, what is crucial in the case of this concept is that the separated charges have a long lifetime on the respective material and closed percolation paths for both types of charge carriers toward the respective contact are present from every location.
U.S. Pat. No. 5,093,698 discloses the doping of organic materials. By admixing an acceptor-like or donor-like doping substance, the equilibrium charge carrier concentration in the layer is increased and the conductivity is increased. According to U.S. Pat. No. 5,093,698, the doped layers are used as injection layers at the interface with respect to the contact materials in electroluminescent components. Similar doping approaches are analogously expedient for solar cells as well.
The literature discloses various possibilities for realization for the photoactive i-layer. Thus, the latter can be a double layer (EP0000829) or a mixed layer (Hiramoto, Appl. Phys. Lett. 58, 1062 (1991)). A combination of double and mixed layers is also known (Hiramoto, Appl. Phys. Lett. 58, 1062 (1991); U.S. Pat. No. 6,559,375). It is likewise known that the mixing ratio differs in different regions of the mixed layer (US 20050110005), or the mixing ratio has a gradient.
Furthermore, tandem and multiple solar cells are known from the literature (Hiramoto, Chem. Lett., 1990, 327 (1990); DE 102004014046).
Organic tandem solar cells have already long been known from the literature (Hiramoto, Chem. Lett., 1990, 327 (1990). In the tandem cell from Hiramoto et al., a 2 nm thick gold layer is situated between the two single cells. The task of said gold layer consists in providing for a good electrical connection between the two single cells: the gold layer brings about an efficient recombination of the holes from one subcell with the electrons from the other subcell and thus has the effect that the two subcells are electrically connected in series. Furthermore, like any thin metal layer (or metal cluster) the gold layer absorbs part of the incident light. This absorption is a loss mechanism in the tandem cell from Hiramoto since less light is thereby available to the photoactive layers (H2Pc (metal-free phthalocyanine)/Me-PTC (N,N″-dimethylperylene-3,4,9,10-bis(dicarboximide) in the two single cells of the tandem cell. In this tandem structure, therefore, the task of the gold layer is purely on the electrical side. Within this conception, the gold layer should be as thin as possible or completely omitted in the best case.
Furthermore, the literature discloses organic pin-tandem cells (DE 102004014046): the structure of such a tandem cell consists of two pin single cells, wherein the layer sequence “pin” describes the succession of a p-doped layer system, an undoped photoactive layer system and an n-doped layer system. The doped layer systems preferably consist of transparent materials, so-called wide-gap materials/layers, and in this case they can also be partly or wholly undoped or else have in a location-dependent manner different doping concentrations or have a continuous gradient in the doping concentration. Especially even very lightly doped or highly doped regions in the boundary region at the electrodes, in the boundary region with respect to some other doped or undoped transport layer, in the boundary region with respect to the active layers or in the case of tandem or multiple cells in the boundary region with respect to the adjacent pin- or nip-subcell, i.e. in the region of the recombination zone, are possible. Any desired combination of all these features is also possible. Of course, such a tandem cell can also be a so-called inverted structure (e.g. nip-tandem cell. All these possible forms of realization for tandem cells are designated by the term pin-tandem cells hereinafter. One advantage of such a pin-tandem cell consists in the fact that the use of doped transport layers enables a very simple and at the same time very efficient possibility of realization for the recombination zone between the two subcells. The tandem cell has e.g. a pinpin-structure (or else e.g. possibly nipnip). An n-doped layer and a p-doped layer are respectively situated at the interface between the two pin-subcells, and form a pn-system (or np-system). A very efficient recombination of the electrons and holes takes place in such a doped pn-system. The stacking of two pin single cells thus directly produces a complete pin tandem cell, without still further layers being required. It is especially advantageous here that thin metal layers, as in the case of Hiramoto, are no longer required in order to ensure efficient recombination. As a result, the loss absorption of such thin metal layers can be completely avoided.
The central problem in optimizing the efficiency of tandem cells consists in the fact that both subcells are intended to generate as far as possible an identical amount of photocurrent. Since highly efficient organic solar cells have a high internal quantum efficiency (almost all photons are converted into electric current), this means that both subcells are intended to absorb light (i.e. number of photons) of the solar spectrum as identically as possible. This is because if one subcell absorbs more light than the other subcell, then the first subcell could actually generate a larger photocurrent than the second subcell. Since the two subcells are electrically connected in series in the tandem cell, however, the current of the tandem cell is always limited by the lower current of one of the two subcells. The potentially larger current of a subcell that absorbs more light thus has to remain unused. Tandem cells therefore have to be optimized such that both subcells absorb as much light as possible and absorb an identical amount of light.
The absorption can be balanced e.g. via the variation of the thicknesses of the two photoactive layer systems. A further possibility in the case of pin-tandem cells consists in positioning the photoactive layer systems in the maxima of the optical field distribution of the light by means of the variation of the thicknesses of the transport layers (this is likewise described in DE 102004014046).
However, the possibilities for adaptation by means of these two methods mentioned are restricted or associated with loss: thus, in a tandem cell, for example, equality of absorption can be achieved by virtue of the fact that, in the “better” subcell, the thickness of the photoactive system is reduced and this subcell therefore absorbs less light, namely just as much as the other subcell. Consequently, although the tandem cell has nominally been optimized, this has also only lead to the fact that the “weaker” subcell in turn limits the component and the potential of the “better” subcell cannot be used. Furthermore, tandem cells which are intended to have a high efficiency have to comprise different absorber systems, i.e. the two subcells contain a plurality of different absorber materials and absorb partly or wholly in different spectral ranges of the light. However, the distribution of the absorption maxima of the light within the component is dependent on the wavelength. This has the effect that in this case the optimization of the thin-film optics for each individual absorber in each of the two tandem cells is very complicated and can only be effected to a limited extent by variation of the thicknesses of the layers (since the different conditions for the individual absorbers generally cannot be fulfilled simultaneously with a set of layer thicknesses).
A further problem for application consists in the fact that solar cells are intended to be used at different locations and under different conditions and the spectrum of the light is therefore different for different applications. Thus, e.g. the light spectrum for applications on roofs corresponds very well to the standard solar spectrum AM1.5 (for central Europe). For house façades integrated systems in towns and cities (especially within narrow urban canyons), however, the conditions are already different and at the latest in indoor applications the available light is completely dependent on the artificial light source. The problem therewith is that the entire optimization of the tandem cells can only ever be effected for a specific light spectrum. For the applications it is thus important to have a simple possibility—which is practical for production—of adapting the tandem cells to different light spectra, without this necessitating greatly changing the construction of the tandem cell or using different absorber materials for each application.
Besides optimizing the luminous efficiency, a further problem consists in the fact that the organic solar cells used are intended to be applied on flexible substrates, such as films, for instance.
The problem here is that although there are very good encapsulation possibilities (e.g. glass-glass encapsulation), for many applications the latter are too expensive and often not flexible. An inexpensive encapsulation that is as flexible as possible is not perfect, i.e. it does not hermetically seal the component completely, rather e.g. water and oxygen gradually penetrate into the cell. A resultant requirement made of the cell is that the latter as far as possible is already itself intended to be very stable toward air and other atmospheres. The aim is therefore to increase the lifetime and to realize an improved stability by virtue of a corresponding encapsulation. At the same time, the intention is to specify a cost-effective possibility for lengthening the lifetime of organic solar cells.
It is known from the literature that metal oxide layers are used as contact layers (Cattin et al., JOURNAL OF APPLIED PHYSICS 105, 034507 (2009); Kim et al., APPLIED PHYSICS LETTERS 95, 093304 (2009). However, increased stability of the components is not mentioned here.
US 2007/0221926 A1, by contrast, discloses a TiOx layer which was applied as a passivation layer to the organic material of a photoactive, organic component and leads to an increased lifetime of the solar cell.